How to Make It
In a bowl, whisk the thyme with the paprika, sage, ground pepper and 1 tablespoon of sea salt. Rub the spice mixture all over the brisket and let stand at room temperature for 1 hour.
Preheat the oven to 300°. In a large flameproof roasting pan set over 2 burners, heat the oil. Add the brisket to the roasting pan and cook over moderately high heat, turning once, until browned, about 8 minutes. Transfer the brisket to a large baking sheet.
Add the onions and a generous pinch of salt to the roasting pan. Cook over moderate heat, stirring occasionally, until the onions are softened and well browned, about 20 minutes. Add the beef stock, vinegar, tomato puree, garlic, peppercorns and bay leaf and bring to a simmer. Return the brisket to the roasting pan, then nestle the fennel in the braising liquid around it.
Tent the brisket with foil and bake for about 6 hours and 30 minutes, until very tender. Transfer the brisket to a carving board, tent with foil and let rest for 20 minutes.
Skim the fat off the braising liquid and discard the bay leaf. Carve the brisket and transfer to a platter. Serve with the pan juices and vegetables.